This invention relates generally to variable Venturi carburetion systems for supplying a fuel-air mixture to the internal combustion engine of an automotive vehicle, and more particularly to a closed-loop fluidic control servo system for automatically regulating the flow of fuel and air admitted into a variable Venturi structure to maintain a desired ratio thereof under varying conditions of load and speed in order to attain higher combustion efficiency, significantly increased fuel economy and reduced emission of pollutants.
The function of a carburetor is to produce the fuel-air mixture needed for the operation of an internal combustion engine. In the carburetor, the fuel is introduced in the form of tiny droplets in a stream of air, the droplets being vaporized as a result of heat absorption in a reduced pressure zone on the way to the combustion chamber whereby the mixture is rendered inflammable. In a conventional carburetor, air flows into the carburetor through a Venturi tube and a fuel nozzle within a booster Venturi concentric with the main Venturi tube. The reduction in pressure at the Venturi throat causes fuel to flow from a float chamber in which the fuel is stored through a fuel jet into the air stream. The fuel is atomized because of the difference between air and fuel velocities.
The fixed sizes of these Venturi's are usually determined by the midrange capacity of the engine. This results in little carburetion action at low capacities for idle and slow speeds so that the carburetion effect is operative only from medium through high speeds. It is for this reason that fuel efficiency is poor at low speeds and the maintenance of the fuel-to-air ratio at all speeds represents a compromise dictated by these limitations.
Another popular carburetor uses manifold vacuum to operate an air-flow cylindrical valve coupled to a tapered needle fuel valve which is controlled by intake manifold vacuum, fuel being introduced eccentrically into a non-circular, non-Venturi passage.
The behavior of an internal combustion engine in terms of operating efficiency, fuel economy and emission of pollutants is directly affected both by the fuel-air ratio of the combustible charge and the degree to which the fuel is vaporized and dispersed in air. Under ideal circumstances, the engine should at all times burn 14.7 parts of air to one part of fuel within close limits, this being the stoichiometric ratio. But in the actual operation of conventional systems, this ratio varies as a function of operating speed and is affected by changes in load and temperature.
To obtain maximum fuel economy, the fuel-to-air ratio in the mixture should be maintained within close tolerances at or about the stoichiometric air-fuel ratio during all modes of operation, such as "idle" while standing still, "slow-speeds" up to about 20 miles an hour, "cruising speeds" and "high speeds." The conventional practice is to provide an accelerating pump system to furnish an extra charge of fuel for accelerations, and power jets or auxiliary barrels for high speed or high power operation, all in addition to the main jet.
Another reason why the maintenance of predetermined fuel-air ratios at or about stoichiometric is important is that the emission of pollutants as well as the power-producing efficiency are in large measure governed thereby. Thus, when the mixture is relatively low in air, carbon monoxide is produced; while when the ratio is excessively rich in fuel, unburned hydrocarbons are emitted in the exhaust.
In modern engine design, the air-fuel ratio in some instances is controlled to maintain a prescribed ratio, or the control system is preprogrammed to accommodate the ratio to specific ranges of speed and load, so that the ratio, for example, is richer at slow speeds and leaner at higher speeds.
The use of an electronic closed-loop engine control system for improving fuel economy and reducing hydrocarbon, NO.sub.3 and carbon monoxide exhaust emissions is known. Thus Niepoth et al. of General Motors, in an article entitled "Closed Loop Engine Control" that appeared in the November 1977 issue of the IEEE Spectrum, disclose a system employing an exhaust gas or "Lambda" sensor in the form of a platinum-coated zirconium element placed directly in the engine exhaust gas stream.
Upon being heated to its operating temperature by the hot exhaust gases, the Lambda sensor acts as an electrochemical cell which generates a voltage as a function of the air-fuel mixture. When this mixture is leaner than stoichiometric, the sensor output voltage is low, the voltage rising in magnitude as the mixture passes through stoichiometric and becomes richer. The signal information yielded by the exhaust sensor is applied to an electronic control unit that processes this information and converts it into a corresponding vacuum signal in a vacuum modulator. A carburetor receives the vacuum signal from the modulator and controls the ratio of fuel-to-air fed to the engine accordingly.
In the General Motors closed loop system, a vacuum regulator at one end of the modulator maintains vacuum at about 6 inches mercury. At the other end is an on-off valve which supplies clean air into the control vacuum port in its "off" position (zero inch Hg vacuum), and which in its "on" position couples the regulated vacuum to this port. By rapid solenoid switching and control of the relative "on" and "off" periods, pulse width modulation of the vacuum is effected. This serves in the carburetor to control the fuel or air in the mixture by means of a vacuum-operated needle-type fuel valve and a vacuum-operated air bleed valve. The engine receives the air-fuel mixture from the carburetor through the intake manifold, the engine burning the mixture and discharging it down the exhaust manifold past the exhaust gas sensor, thereby closing the loop.
A closed loop system of the General Motors type suffers from distinct limitations, for it fails to take into account certain factors essential to efficient carburetion. It is, of course, desirable to correctly proportion the amount of fuel-to-air in the mixture to be burned in order to fully utilize the fuel. But if the fuel in the correctly proportioned mixture assumes the form of large droplets and is not adequately vaporized before being admitted into the engine, full combustion will not result and the exhaust will include unburned hydrocarbons and carbon monoxide which must be cleaned up in exhaust gas treatment apparatus such as air pumps and converters.
It is vital, therefore, that the carburetor function to fully atomize and disperse the fuel to supply a homogenized, uniformly distributed vaporized mixture into the intake manifold of the engine; for only in this way will the engine operate with optimum fuel economy and with minimal exhaust pollutants.
In the General Motors closed loop carburetor engine system and in the Bosch, Lucas and other fuel injection systems, wherein the operation of a carburetor or fuel injector is pulsemodulated in order to attain the desired fuel-to-air ratio, the inherently intermittent action required by this system lends itself to electronic control. But an intermittent action is incompatible with hydrodynamic gasifying requirements and gives rise to poor fuel atomization and dispersion, so that a properly gasified mixture is not yielded thereby.
In the above-identified copending application of which the present case is a continuation-in-part, there is disclosed a closed-loop engine control system which maintains that ratio of air-to-fuel which represents the optimum ratio for the prevailing condition of engine speed and load. The system includes a variable-Venturi carburetor which acts to properly atomize and disperse the fuel in the air whereby the system not only brings about a marked improvement in fuel economy but also substantially reduces the emission of noxious pollutants. The disclosure of this copending application and the still earlier patent applications related thereto are incorporated herein by reference.
In the system disclosed in my copending application, the variable Venturi structure is constituted by a cylindrical casing and a cylindrical booster coaxially disposed therein whose internal surface has a Venturi configuration to define a primary passage. Interposed between the booster and a section of the casing wall having an external Venturi configuration is an axially-shiftable spool whose internal surface has a Venturi configuration to define between this surface and the spool a variable secondary passage whose throat size depends on the axial position of the spool. A tertiary passage is defined between the outer surface of the spool and the casing section. Air passing through the casing flows through all three passages.
An air-fuel dispersion is fed by a nozzle into the primary passage to intermingle with the air flowing therethrough to form an atomized mixture which is fed into the second passage to intermingle with the air flowing through the throat thereof, from which secondary passage the mixture is fed into the intake manifold of the engine.
The differential air pressure developed between the inlet of the Venturi structure and the throat of the tertiary passage therein is sensed to produce an air-velocity command signal which is applied to a control module that governs a servo motor operatively coupled to the spool to axially shift the spool and thereby adjust the throat of the secondary passage. The intake manifold vacuum which varies as a function of load and speed conditions is sensed to produce a speed-load signal for modulating the command signal in the control module in a manner maintaining an optimum air-fuel ratio under the varying conditions of load and speed.
The air velocity command sensor and the intake manifold vacuum sensor are provided with operating characteristics that are predetermined for an engine of specified size and its load. Thus the signals from the sensors are "pre-programmed" for the responses desired in the operating modes of the engine. The control module, in essence, is an analog computer that not only responds to the command and speed-load signals to effect one-line control of the air-fuel ratio as pre-programmed, but it also accepts auxiliary signals for presetting and adjusting the control to take into account ambient and exhaust conditions. These auxiliary signals are derived from ambient and exhaust sensors which afford continuous control of these variables in real-time.
The velocity of air flowing through the Venturi structure is governed as a function of air volume by a closed process control loop whose air velocity command signal is modulated by a speed-load signal reflecting the degree of intake manifold vacuum developed under the prevailing conditions of speed and load. Thus the air velocity pressure through the Venturi structure is controlled as a function of air-volume and the velocity-pressure controls fuel volume, thereby providing a controlled fuel-air ratio during all changes in air flow as determined by throttle position and engine response. In this way, the "fuel loop" is directly controlled by the "air loop" and the flow of air and fuel in the structure are correlated to cope with transitions through the various modes of vehicle operation smoothly and without hesitation within the prescribed desirable ratios. The emission of pollutants are then held at a low level regardless of the mode of operation .
The system disclosed in my copending application includes at least two transducers: one acting to convert the pressure differential existing between the inlet and throat of the variable Venturi into a command signal proportional thereto; the second acting to convert intake manifold negative pressure into a speed-load signal. These two transducers, in combination with the electronic circuit module and the servo-motor operated thereby to vary the axial position of the spool in the variable Venturi carburetor, constitute the main components of a closed loop electronic control system operating, in real time, to maintain the appropriate air-to-fuel ratio for all operating phases of the engine and the load imposed thereon.
The effectiveness and reliability of this electronic control system obviously depends on the accuracy and durability of the electronic and electrical components included therein. In this age of sophisticated technology, there is a tendency to regard the replacement of a pneumatically or mechanically operated system with an electronic equivalent as a technical advance, and in many instances this view is justified.
However, in the real world of internal combustion engines, personnel trained to operate, maintain and repair such engines are generally not qualified to copy with electronic systems. Though a conventional internal combustion engine which breaks down in the field can often be repaired with simple tools, where the engine includes an electronic control system and the breakdown is due to defective transducers or a disabled electronic module or microprocessor, there is usually little that can be done in the field to overcome this problem.
Moreover, an electronic control system in the environment of an internal combustion engine is subjected to temperature extremes, vigorous vibrations and mechanical shocks, fumes, oil and other conditions for which typical electronic components are not prepared. These conditions create maintenance problems and often shorten the effective life of an electronic system.